A dampening unit of this type is known from DE 34 16 845 Al. In this reference, a dampening unit generally includes a feed device for the dampening medium, a device for metering the dampening medium film and an applicator roller which transfers the dampening medium film onto a plate cylinder and which can be coupled to an adjacent inking unit or operated separately. The dampening applicator roller can be operated at a different circumferential speed than the plate cylinder, so that a resulting wiping effect eliminates foreign particles.
German patent publication DE 34 32 807 Al, in conjunction with U.S. Pat. No. 4,724,764, discloses a more refined dampening unit. In addition to selectively operating the plate cylinder and dampening applicator roller at different circumferential speeds, an ink receiving roller, called a rider roller, is provided for the dampening applicator roller. In this case, the ink-receiving roller rotates at a circumferential speed different from the plate cylinder and can be coupled to an adjacent inking unit or be operated separately.
A disadvantage of the foregoing prior art dampening units is that ink accumulating on the dampening applicator roller is carried into the dampening unit and consequently the ink/dampening medium equilibrium can become impaired. These impairments can take the form of ghosting (shadow-like markings) or streaking which cause uneven printing.
A dampening applicator roller in a dampening unit is also disclosed in DE 30 16 366 Al. In particular, this dampening unit includes a metered-feed applicator roller, that acts as metering device and is arranged upstream of the point of contact of the dampening applicator rollers with the plate cylinder. The dampening unit also includes an oscillating cylinder having a hydrophilic surface which is arranged downstream of the contact point of the dampening applicator rollers and the plate cylinder.
DE 195 29 204 Al discloses yet another dampening unit for an offset printing machine. The disclosed dampening unit includes a dampening applicator roller which is movable into and out of contacting relation with the plate cylinder. Additionally, a dampening medium feed device is arranged upstream of and in contact with the dampening applicator roller in the direction of rotation and a roller that accepts dampening medium is arranged, as a rider roller, downstream of the dampening applicator roller. Further, an ink-carrying rider roller is arranged downstream of the rider roller and also is in contacting relation with the dampening applicator roller.